Digital lighting technologies, i.e. illumination based on semiconductor light sources, such as light-emitting diodes (LEDs), offer a viable alternative to traditional fluorescent, HID, and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, durability, lower operating costs, and many others. Recent advances in LED technology have provided efficient and robust full-spectrum lighting sources that enable a variety of lighting effects in many applications. Some of the fixtures embodying these sources feature a lighting module, including one or more LEDs capable of producing different colors, e.g., red, green and blue, as well as a processor for independently controlling the output of the LEDs in order to generate a variety of colors and color-changing lighting effects, for example, as discussed in detail in U.S. Pat. Nos. 6,016,038 and 6,211,626.
Typically, an LED-based lighting unit or LED load that includes multiple LED-based light sources, such as a string of LEDs connected in series, is driven by a power converter, which receives voltage and current from mains power supply. To reduce driver cost, the LED load may be driven directly from the mains power supply, as an alternative, including AC and DC operation. However, there are drawbacks related to AC driving directly from the mains power supply. For example, the current waveform provided to the LED load has a high peak value compared to the average value. Therefore, the LED load is driven with a reduced efficiency due to droop, as well as a low power factor. Also, current flow is only possible when the instantaneous mains voltage is higher than the forward voltage of the LED load. Therefore, there may be relatively long periods during which no current flows to the LED string and no light is produced, causing flicker.
To partially address these issues, a rectifier circuit may be connected between the mains power supply and the lighting unit, and a capacitor may be connected in parallel with the LED load within the lighting unit. For example, FIG. 1 illustrates a circuit diagram of a conventional LED-based lighting unit 100, which includes bridge rectifier circuit 110, LED load 160 and capacitor 141, which acts as a power factor control (PFC) and smoothing circuit 140. The capacitor 141 is connected in parallel with the LED load 160, which includes resistor 163 connected in series with a string of one or more LED light sources, indicated by LEDs 161 and 162. The bridge rectifier circuit 110 is connected to mains power source 101 via resistor 105, and includes diodes 111 to 114. The bridge rectifier circuit 110 thus outputs a rectified mains voltage or input voltage Urect to the circuit 140.
However, due to the charging and discharging waveform of capacitor current IC input to the capacitor 141 and the shape of the mains voltage waveform, the LED-based lighting unit 100 typically consumes current, e.g., to recharge the capacitor 141, within a relatively short time period, resulting in high current peaks and a low power factor. In addition, predominantly the resistor 105 connected to the mains power source 101 limits both the repetitive and the initial charging of the capacitor 141. Therefore, when the LED load 160 is initially turned on, there may be an excessive in-rush current. For example, if the LED load 160 is turned on during a mains voltage peak of the mains power source 101, the capacitor current IC of the capacitor 141 may be relatively large, as compared to nominal operation. As a result, unless LED load 160 includes several light sources connected to one circuit in series, resulting in a relatively low value of the nominal LED operation current, due to the further components in the LED-based lighting unit 100, already a relatively small number of light sources will be enough to trigger a magnetic release of the circuit breaker. Therefore, the number of LED-based lighting units 100 connectable to one circuit may be dramatically lower (e.g. only 1/10 or even 1/50) than one may expect according to the nominal current.
From efficiency point of view, and when looking at an individual LED-based light source, the waveform of the current does not present a problem. However, when locking at a large number of LED-based light sources, high currents during a short time interval create distortion on the mains grid and may trigger a circuit breaker (e.g., trigger a fast acting magnetic release of a circuit breaker). Due to the mains distortion, use of LED loads with very low power factors is prohibited by regulation. For example, in Europe, the required power factor may be as low as 0.5, which is attainable using the rectifier and capacitor solution, described above. However, other regions require relatively high power factors, such as 0.7 or higher, e.g. 0.9.
Thus, there is a need in the art to AC drive LED-based lighting units directly from the mains power supply, while maintaining relatively high power factors. In addition, there is a need in the art for preventing excessive in-rush currents when initially turning on LED-based lighting units driven directly from the mains power supply.